Chapter Five

Note

If you are totally new to programming, please make sure you have completed Chapter One, Chapter two, Chapter Three and Chapter Four.

Dividing Your Code into Blocks

Up to now, we have learned all the basics principles of programming that will help us start building our own programs. However, as you start building your own programs, your codebase will grow larger as you keep adding new functionalities. This makes your code hard to manage and modify. To address this problem, we need to properly organize our code. We can do this by dividing our code into different independent parts. Avoid repeating yourself by writing the same code and the same solution again and again. Instead create reusable pieces of your code that can be used whenever needed. Let's write a program to demonstrate this concept.

Create a new file named blocks.py and write this code:

def greet():
    print("Hello world!")

greet()
greet()

a = 10
print(a)
greet()

Run the program by using python blocks.py command in the Terminal.

Output:

Hello world!
Hello world!
Hello world!
10
Hello world!

This program prints "Hello world"! multiple times and print a variable at some point. However, in this program, we used a new technique to move the print("Hello world!") part of our program into a block that starts with def greet(). This is called function. A function allows us to group a piece of code into a specific block, name that block, and use the name of the block wherever we need it to run the same code. This will help us reuse the code without writing it repeatedly.

We create a function by using def keyword followed by the name of the function, which can be anything, and then a pair of parentheses, this is called definition of a function. Wherever we want to use the code inside the function we call it by writing the name of the function followed by a pair of parentheses, this is called calling a function. In our code, we defined a function named greet and placed our code print("Hello world!") inside it. Then we used the name of the function to tell the program to run he code inside its code block. By calling the function multiple times we printed the text Hello world! multiple times.

Passing Data to a Function

When calling a function we can give it some values to use it in its code block.

Create a new file named functions.py and write this code:

def greet(name):
    print("Hello " + name)

greet("John")
greet("Ali")
greet("My Friend!")

Run the program by entering python functions.py command in the Terminal.

Output:

Hello John!
Hello Ali!
Hello My Friend!

This program defines a function that takes value in the parentheses. We then call the function three times, passing it different values. Each time we pass a new value, it construct a new message accordingly and print it. The value we pass into the function is called an argument.

When defining a function, we specify how many value could passed to it. We list names for those values in the parentheses during the definition of the function. The value we passed in our program is named name, but we can name it anything.

We can define multiple values to be arguments to our function. Take a look at the following different examples of functions:

def add(a, b):
    result = a + b
    print(result)

add(4, 4) # will print: 8
add(3, 8) # will print: 11
add(a=3, b=8) # will print: 11
add(3, b=8) # will print: 11
add(a=3, 8) # will print: 11

def multiplyThree(a, b, c):
    result = a * b * c
    print(result)


list1 = [1, 3, 4]
multiplyThree(list1[0], list1[1], list1[2]) # prints: 12

# Passing a list into a function
def addAll(the_list):
    result = 0
    for item in the_list:
        result = result + item

    print(result)

addAll(list1) # will print: 8

Getting a Value from a Function

Function can return a value to us, not just receive a value from us. To demonstrate the concept, let's write a program.

Cerate a new file named functions2.py and write this code:

def add(a, b):
    result = a + b
    return result

c = add(10, 5) # Function add 10 + 5 and return 15. c is 15 now
print(c) # prints: 15

Run the program by entering python functions2.py in the Terminal.

Output:

15

Our program here, we first calls a function, named add and pass it two values 10 and 5. The function, then adds 10 and 5 and place the result in the variable result. The function then returns the value of variable result to where it was called from. So the the variable c receives the value from the function and becomes 15. Finally the program prints the value of variable c.

To return a value from a function, we use the keyword return followed by whatever we want to be returned.

Summery

In this chapter, we learned how to organize our code into blocks by using functions. We then learned how to define and call a function. We demonstrated that how functions can save us from repeating ourselves. After that, we discoverd how to pass values to functions so they can use it in their code blocks. Lastly, we learned how to return a value from a function.

Exercise

Rewrite all the codes in this chapter and experiment with them. And make sure you do it. Because all exercise are important and they will help us learn things faster and better.